\begin{tikzpicture}
    \tkzDefPoints{0/0/O}
    \tkzDefPoint(180:1.5){A}
    \tkzDefPoint(0:1.5){B}
    \tkzDefPoint(40:1.5){C}
    \tkzDefPoint(-40:1.5){D}
    \tkzInterLL(A,B)(C,D)  \tkzGetPoint{P}

    \tkzDrawCircle[thick](O,A)
    \tkzDrawPoint(O)
    \tkzDrawSegments(A,B  C,D)
    \tkzMarkRightAngle[size=.2](B,P,C)
    \tkzLabelPoints[above](O)
    \tkzLabelPoints[left](A)
    \tkzLabelPoints[right](B)
    \tkzLabelPoints[above, xshift=.3em](C)
    \tkzLabelPoints[below, xshift=.3em](D)
    \tkzLabelPoints[below left](P)
\end{tikzpicture}

